For humans its mugs and chocolate bars, for anthropoids its frozen fruit juice sticks and peanut butter

One of the seminal findings of behavioral economics is the endowment effect, in which humans show a tendency to value a good they have just come to possess more than the maximum price they would have paid immediately before they obtained it.. Turns out the endowment effect may be a simian tendency too. (Ok, that’s all the appropriate synonyms we know, from now it’s chimpanzees.)

The famous standard endowment effect experiments involved randomly handing out mugs and chocolate bars. A group of researchers tried a similar experiment with 33 Texas chimps, frozen fruit juice sticks and peanut butter stuffed into the end of a PVC pipe. As a variation of the experiment, researchers also introduced non-food items into the exchange like a rubber bone and knotted rope dog chew toy

As a group, 58% of the chimpanzees preferred (peanut butter) to juice. However, when endowed with the (peanut butter), 79% of the chimpanzees preferred to keep the (peanut butter) rather than exchange for juice, approximately 20% more of the population than would be expected from the population-wide preference. Likewise, when endowed with juice, 58% of the chimpanzees chose to keep the juice rather than exchange for peanut butter , reflecting an endowment effect in approximately 15%.

For comparative purposes, the endowment effect was actually stronger in humans. In the original mug/chocolate bar experiments, 33 percent more people kept their mugs and 46 percent more kept their chocolate bars than expected given everyone’s original preferences.

When given non-food items, however, the chimpanzees showed a greater willingness to trade. Experimenters ran the experiment in six trials in order to vary the food and chew toy options. In the clearest sign of how the intensity of the endowment varied between categories, 14 times as many chimpanzees kept possession of both foods they initially received as chimpanzees who kept possession of both chew toys, (42 percent to 3 percent). When the experiment involved food and non-food items together, chimps were more likely to swap the non-food items. The authors comment on the reasons behind these differences.

Three features of chimpanzee ecology may help explain the differences in endowment effect prevalence between foods and non-foods. The first is that foods have more significant effects on fitness than non-food items. Second, chimpanzees do not show long-term possession or storage, so items neither accumulate nor have value outside their immediate utility. Third, chimpanzees lack the reliable, institutionalized, third party bargain enforcement mechanisms that humans have. This renders each exchange inherently risky, as there can be no guarantee that giving up one item will yield another, instead of a total loss. Thus, for chimpanzees, and early humans, there was likely a fitness advantage to maintaining possession of some items even in the presence of superior exchange options, making selectively possessive behavior that appears irrational in the moment rational from an evolutionary perspective. Humans in modern times may still exhibit stronger endowment effects in evolutionarily salient situations, a hypothesis that warrants further empirical investigation in light of the puzzlingly varying
frequencies and magnitudes in which humans exhibit endowment effects and other seemingly paradoxical behaviors.

Those interested in previous monkey research with behavioral economics implications can click here or here to read out about cognitive dissonance, and here to read out about altruism and strategic cooperation.

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The Nudge blog is associated with the book Nudge: Improving Decisions about Health, Wealth, and Happiness, by Richard Thaler and Cass Sunstein. Sunstein is currently the Administrator of the White House Office of Information and Regulatory Affairs and has no affiliation with the Nudge blog.